Add odm sepolicy support to selinux.cpp

init: support loading odm sepolicy

Currently init merges two sepolicy cil files:
    - /system/etc/selinux/plat_sepolicy.cil
    - /vendor/etc/selinux/nonplat_sepolicy.cil

This change replaces nonplat_sepolicy.cil with the following two files:
    - /vendor/etc/selinux/declaration/nonplat_declaration.cil
    - /vendor/etc/selinux/vender_sepolicy.cil

And support merging another default (but optional):
    - /odm/etc/selinux/odm_sepolicy.cil.

Bug: 64240127
Test: boot sailfish normally without odm.cil
Test: boot another device having odm.cil
Change-Id: I0b7f8c656c73ddb0fd46f2af3c625d7c81566f2f